Hitherto, circuit boards have been fabricated such that what is known as the inner layers, i.e. the conductive regions of the circuit board, are built from horizontal as well as vertical sandwiched insulating layers. For this purpose, various processes may be used which are described, for example, in "Handbuch der Leiterplattentechnik", Vol. 2, 1991, E.G. Leuze Verlag, Saulgau, Germany.
Such a substrate, which is normally fabricated by lamination, is coated either unilaterally and bilaterally with a metal, generally a copper foil.
Subsequently, through and/or blind holes are drilled into the metal-coated substrate and cleaned from drilling residue.
Then the holes in the substrate as well as the surface (full panel plate) or only the image pattern (pattern plate) are generally subjected to an electrodeposition of metal in, say, a copper bath. On the surface, metal is thus deposited on the metal foil, whereas in the holes it is applied directly to the substrate consisting of conductor planes and insulating material.
The substrate surface is blanket-coated with photoresist which is exposed and developed to form a pattern, while the region of the holes is "tented", meaning that the holes are covered with photoresist. Thus, the future conductors are defined in the remaining surface regions.
In the next step, the metal, i.e., the electrodeposited layer and the foil, is removed, for example, by etching, and the photoresist is stripped, retaining the metal protected by "tenting" in the holes and that protected by photoresist in the remaining regions.
In the last step, a solder stop mask is applied to the surface of the substrate, exposed and developed. This may be followed by a test and separating steps.
In this manner circuit boards are obtained, wherein the metal layer thicknesses on the circuit board surface and in the holes are dependent on each other. As a predetermined minimum layer thickness of metal must exist in the holes, this process using thick metal layers on the surface (metal foil and electrodeposited metal) is hardly suitable for fabricating fine-line products such as those with conductor widths of less than about 75 .mu.m (about 3 mil).